Build fails due to `relative-date` being removed from NPM

[jvehent]

Following the NPM mass deletion, npm install on persona fails due to relative-date no longer being present.

npm ERR! Error: version not found: relative-date@1.1.1
npm ERR!     at /usr/share/npm/lib/cache/add-named.js:125:12
npm ERR!     at saved (/usr/share/npm/node_modules/npm-registry-client/lib/get.js:167:7)
npm ERR!     at Object.oncomplete (fs.js:107:15)
npm ERR! If you need help, you may report this *entire* log,
npm ERR! including the npm and node versions, at:
npm ERR!     <http://github.com/npm/npm/issues>